DD used here refers to a Doubled Die. During the hubbing process while creating the die, it was hubbed not the same as previously. This causes the die to be doubled. Thus the term Doubled Die. Theses are collectable because the devices are enlarged and most of the time and will show a spread between the hubbings. On the coin it will look like two sets of devices or wider devices depending of the type (class) of doubled die it is. Most of the time you see the letters
DDO (Doubled Die Obverse) or
DDR (Doubled Die reverse) The stronger the spread, the more interest in them.



Because this coin has such a strong spread and it is a proof example, just a few of these were struck. So the price range can be in the thousands for one of these in high grade condition.
Hope this helps.
